Studies of p53 in mesothelioma has revealed positive immunhistochemical staining in cells whose genomic p53 sequence is wild type. In order to evaluate the biological meaning of these observations, normal human mesothelial cells and mesothelioma cell lines were exposed to ionizing radiation. After 4 hours, the normal cells and 5/6 mesothelioma cell lines tested showed 40-60% increase in cells with high, nuclear p53 accumulation. In addition, a subset of those cells also showed an increase in p21WAF1. These studies indicate that the endogenous p53 in mesothelioma is responsive to DNA damage and is consistent with the possibility that it is active as a transcriptional activator. In addition, mesothelioma tumors and cell lines have been studied to establish whether loss of the tumor suppressor gene, p16, is an important step in mesothelial carcinogenesis. Southern blot analysis of 17 tumor samples reveals that 15/20 show a loss of p16. Analysis of normal human mesothelial cells in culture, as well as mesothelioma cell lines shows that p16 is present in all normal cells but is lost in 17/17 cell lines. All cell lines and normal human mesothelial cell samples showed normal Retinoblastoma gene expression with multiple phosphorylated species. These studies suggest that homozygous deletion of p16 is a common step in the pathogenesis of mesothelioma and indicate that alteration of the G1 checkpoint controlled by the Retinoblastoma protein is critical to malignant progression of this tumor type.